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First observation of very long-lived spins in superconductors
-Opening the way to novel superconducting quantum computing devices-

Jun. 7, 2010

Joint research group of JAEA, Tohoku University, and IBM Research-Almaden has discovered that the lifetime of spin-polarized quasiparticles injected into a superconductor from ferromagnetic tunnel spin injectors is unexpectedly long. The lifetime of the spins (akin to particles spinning around their own axis) in the superconducting state is about a million times longer than that in the normal conducting state. This feature is essential to the development of sensors with ultra-high sensitivity and quantum-bits for future super-fast quantum computers.

Spin electronics, often referred to as “spintronics”, is a rapidly emerging nano-technology field of research made possible by recent developments in nanofabrication techniques. Whereas conventional electronic devices use only the electron’s charge, spintronic devices take advantage of the electron’s spin. Typically, ferromagnetic materials play a central role: they provide sources of electrons with spin “up” or spin “down.” On the other hand, superconductors are environmentally friendly materials which lose all electrical resistance at low temperatures and so can carry electrical current without any dissipation of energy. Unfortunately superconductivity is usually incompatible with magnetism and is easily damaged by its contact with a ferromagnet. For this reason, useful spintronic devices incorporating superconductors have not yet been developed.

To overcome this difficulty, the research group has developed high-quality, double barrier tunnel junction devices which consist of an aluminum (Al) superconducting layer, separated from cobalt-iron (CoFe) ferromagnetic layers by thin magnesium-oxide (MgO) tunnel barrier layers (Figure 1). The Al layer becomes superconducting below the absolute temperature of 2.3 K. They measured precisely the tunnel conductance (inverse of resistance), the ratio of the measured tunnel current to the applied bias voltage (Figure 2a), and the relative change in the tunnel resistances between the parallel and antiparallel alignment of the magnetizations of the CoFe electrodes, the so-called tunnel magnetoresistance (TMR), as shown in Figure 2b. These observations indicate that the injected spins are preserved for a very long time in the superconducting state.

In this study, the research group succeeded for the first time to inject and accumulate spins in the superconducting state of a superconductor, and found the extremely long lifetime of spin in the superconducting state. This suggests that spins in the superconducting state are candidates for quantum bits, a basic element of quantum computing, and so paves the way to the development of a new type of quantum bit and quantum information technologies.

These findings have been published in “Nature Materials” as an Advanced Online Publication on June 6th, 2010.

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